Black, Joseph (1728–1799)

Scottish physician, chemist, and physicist, who helped establish
the distinction between heat and temperature,
and introduced the concepts of specific
heat and latent heat. Joseph Black
was born in Bordeaux, France, to parents of Scottish descent. After spending
six years at school in Belfast, he entered the University of Glasgow in
1746 to study chemistry and medicine. In 1756 he became professor of anatomy
and lecturer in chemistry at Glasgow. Ten years later he succeeded to the
chair of medicine and chemistry in the University of Edinburgh.

Black was popular with his students because he took great pains in planning
his courses and because his lectures were illustrated with many experiments.
As well as making a number of valuable contributions to both physics and
chemistry, he also found time to practice as a physician.

Joseph Black and the measurement of heat

The difference between temperature and heat is now well established even
though it is easy to be confused by these two related terms. Temperature
(the intensity of heat) can be measured quite easily using a thermometer.
In contrast, the measurement of heat (quantity of energy) is more complicated.

Joseph Black carried out a series of important experiments concerning the
measurement of heat and the relationship between heat and temperature. He
showed that although there was no change in temperature, large quantities
of heat were absorbed in melting ice and in turning boiling water to steam.
He introduced the term latent heat for the heat required to bring about
these changes of state.

Black also found that different amounts of heat were required to produce
the same temperature rise in equal masses of different substances. For instance
about 1.7 times more heat is required to raise the temperature of water
from 15°C to 25°C than to bring about the same increase in temperature
in an equal mass of ethanol (ethyl alcohol). To explain this variation between
different substances, Black introduced the idea of specific
heat. While carrying out this work he laid down the principles of heat
measurement – calorimetry – which are still used today.

Black's discoveries in chemistry

For the previous hundred years or more, advances in chemistry had been hampered
by the phlogiston theory. However, as Black was suspicious of theories which
were not supported by experimental evidence, he was able to make several
valuable additions to chemical knowledge.

Until the middle of the 18th century very little was known about gases,
in fact many people thought there was only one gas – air. About a
century before, in 1640, van Helmont had discovered the gas which is now
known as carbon dioxide, but with
the rise of the phlogiston theory the
significance of this was lost.

Black rediscovered carbon dioxide in 1754 while experimenting with two mild
alkalis – the carbonates of magnesium and calcium. he found that when
these compounds were heated, each produced a stronger alkali while at the
same time fixed air – carbon dioxide – was liberated. The weight
of the stronger alkali was less than that of the mild alkali from which
it was obtained.